The present invention relates to a camshaft for internal combustion engines, and more particularly to a camshaft in which the cams are attached by screws.
The camshaft of an engine normally supports several cams. Between the cams, the camshaft must be supported by bearings. With conventional, undivided bearings the camshaft needs to be slipped axially into the casing and, for reasons of assembly, must at the bearing points have at least the diameter on which the cam apexes revolve. Bearing diameters of that size result in high bearing friction, substantial bearing backlash with correspondingly inaccurate guidance of the camshaft, and excessive construction space requirements in the region of the bearings.
When giving the camshaft at the bearing points a considerable smaller diameter for load-bearing reasons, friction and backlash problems turn out to be considerably more favorable. But the camshaft must then be installed radially, which calls for divided bearings having considerably higher engineering expense. Owing to the necessary screw joints, the construction space requirement tends to be higher than it is with integral bearings.
The disadvantages of the above two camshafts bearing support designs also occur with assembled camshafts, where individual cams with a single cam nose each are pinch-fitted on a shaft or secured to the shaft by hydraulic expansion. The advantage of such assembled camshafts resides in the option of using different materials for the shaft and for the cams.
The use of undivided bearings with a small diameter is obviously possible only with a camshaft allowing disassembly, with the shaft being able to be axially inserted, in the bearing bores and in the cams arranged between the bearings. This offers at the same time the advantage that different materials may be used for shaft and cams.
German Patent Document DE 33 17 019 describes a control shaft in large-scale engine building, that enables pinch-fitting of cams on a cylindrical shaft by means of conic sleeves, which both with the shaft and the cams enter into a compression joint.
While the construction space requirement in the radial direction may be viewed as limited, it tends to increase in axial direction as compared to the conventional solution. For one, the conic sleeve itself must have a sufficient length for centering the cam nose and for the safe transmission of the torque, and for another there is an axial stop required for the sleeve, in order to prevent the cam from jumping off the cone as hydraulic fluid is introduced to achieve a definitive fit. This sets limits to the use of this solution in more compact internal combustion engines, for instance passenger car engines.
Further disadvantages of the solution described in German Patent Document DE 33 17 019, however, evidence themselves especially in the expensive assembly operation. Specifically, an unequivocal and sufficiently accurate positioning of the cams in an axial direction as well as in a peripheral direction is possible only with appropriate assembly devices.
German Patent Document DE 39 43 426 describes a camshaft where cam elements are each secured on a smooth shaft by a cylindrical cross pin. Since in production it is difficult to measure and ream the cross bore in the cam element together with the cross bore in the shaft after installation of the cam element, very high tolerance requirements are needed on the positional accuracy of the separately machined bores. Additionally, owing to the relatively small nominal dimension, extremely small diameter tolerances must be specified both for the bores and the pin. Both such specifications lead to very high manufacturing costs. Additionally, with the cross pin attached, it is not possible to maintain a surface pressure between the surface of the shaft and the bore in the cam element. Therefore, the changing torque is transmitted strictly positively, and alternating bending deformation of the transverse pin that leads to constant relative motions and reduced durability of the joints.